Reliable detection of multi-level data in packet switched systems is difficult enough (as detailed below) without the complication of carrier frequency offset errors. The instant invention eliminates the frequency offset complication from the data detection process.
FM receivers often employ automatic frequency control (AFC) to adjust their own center frequency to track the carrier frequency. Standard AFC techniques are far too slow to adapt to the rapidly and ever-changing environment of fast packet switching among a multitude of transmitters, and if made fast enough, would track out the very data of interest, particularly when encountering consecutive identical symbols. Dual time constant AFC techniques (those employing fast attack and normal decay time constants) could be employed, but add considerable expense and complexity.
Television receivers are believed to employ DC restoration techniques that "center" their light-to-dark ratios with reference to maximum-amplitude synchronizing pulses received representing the blackest black; but the present invention requires no such pulses or any reference data whatsoever to "center" its data detector.
The invention then takes as its object to overcome these shortcomings and to realize certain advantages presented below.